X-ray imaging is a powerful tool in many fundamental and practical applications. As a primary example, X-ray computed tomography (CT) is a cornerstone of modern hospitals and clinics. The dominating theory of X-ray imaging is generally based on the attenuation contrast mechanism.
X-ray gratings have been used for hybrid CT imaging in terms of attenuation, refraction, and small-angle scattering. This grating-based approach represents a paradigm shift in X-ray CT from gray-scale (attenuation) to true-color (attenuation, refraction, and small-angle scattering, which is also referred to as dark-field, and spectral) imaging.
In conventional X-ray imaging, the image contrast arises from varying linear attenuation coefficients. Attenuation-contrast-based imaging exhibits good performance only when strong attenuators are embedded in a weakly absorbing matrix, such as in the cases of bone-tissue and tissue-air interfaces. However, biological soft tissues include mainly light elements (e.g., hydrogen, carbon, nitrogen, and oxygen), and their compositions are quite homogeneous with little density variation. The attenuation-contrast between soft tissue features is often insufficient to reflect pathological changes.
In particular, many healthy tissues display similar characteristics in current X-ray images as those of diseased tissues, such as tumors. For example, fibro-glandular tissue can have a density of 1.035 cm−3 and an attenuation coefficient of 0.80 cm−1, and cancerous tissue can have a density of 1.045 cm−3 and an attenuation coefficient of 0.85 cm−1. Given inherent measurement noise, it is challenging to discern such cancerous tissue from the healthy tissue, as well as other soft tissue features such as those reflecting musculoskeletal healing. Therefore, attenuation-contrast-based imaging is unable to differentiate early-stage tumors and soft tissues.
Use of X-ray gratings can provide for not only attenuation but also phase-contrast and dark-field information. An X-ray grating-based imaging approach typically includes an ordinary X-ray source, and a source grating (often known as G0), and a phase grating (often known as G1) and an analyzer grating (often known as G2) are also typically used. The main purpose of the source grating is to provide sufficient spatial coherence for differential phase-contrast imaging. The source grating can have a micrometer-range period, such as a period of 50 μm or about 50 μm. It is difficult, however, to make a high-quality and large area source grating.